Add runtime-env for graalvm native-image support

[cap10morgan]

I added core/runtime-env to better support graalvm native-image (where core/env reflects the compile time environment).

Since it's a delay you just deref it:

(require '[environ.core :as environ])
@environ/runtime-env

...and it returns the same data structure as core/env but reflects the runtime config environment.

It is a delay so that you it loads the config the first time you deref runtime-env but then caches it after that (like env does for compile time config).

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[cap10morgan]

This is an alternative solution to #92

[SerhiiKozachenko]

As an option, maybe use: *compile-files* flag, https://groups.google.com/g/clojure/c/hhih2Ty7-Lc/m/yXa43-X441gJ

For example:

(defonce ^{:doc "A map of environment variables at runtime."}
  env (when-not *compile-files* (read-env)))

Nevermind, that doesn't help with native-image.